Chapter 1: Mendel's Principles of Heredity

gene

a unit of heredity that affects a trait

allele

different versions of the same gene

genotype

the genetic makeup of an organism (alleles present)

phenotype

the observable traits of an organism

homozygous

having two identical alleles for a gene

heterozygous

having two different alleles for a gene

dominant allele

an allele that is expressed in the heterozygous condition

recessive allele

an allele that is only expressed when homozygous

law of segregation

the two alleles for a gene separate during gamete formation

law of independent assortment

alleles of different genes assort independently into gametes (if unlinked)

monohybrid cross

a genetic cross involving one gene

dihybrid cross

a genetic cross involving two genes

Aa x Aa phenotypic ratio (complete dominance)

3 dominant : 1 recessive

Aa x Aa genotypic ratio

1 AA : 2 Aa : 1 aa

Aa x aa ratio

1 dominant : 1 recessive

AaBb x AaBb phenotypic ratio

9:3:3:1

AaBb x aabb phenotypic ratio

1:1:1:1

testcross

crossing an individual with a dominant phenotype to a homozygous recessive individual

purpose of a testcross

to determine the genotype of an individual showing a dominant phenotype

result of Aa x aa

50% dominant phenotype, 50% recessive phenotype

product rule

probability of two independent events occurring simultaneously can be calculated by multiplying the individual probabilities of each event occurring alone

sum rule

probability of one event OR another occurring is found by adding probabilities

independent events

events where the outcome of one does not affect the other

number of different gametes formula

2^n (n = number of heterozygous gene pairs)

homozygous gene pair contribution to gametes

produces only one type of allele

heterozygous gene pair contribution to gametes

produces two possible alleles

incomplete dominance

neither allele is completely dominant; heterozygote shows intermediate phenotype

phenotypic ratio in incomplete dominance (Aa x Aa)

1:2:1

codominance

both alleles are fully expressed in the heterozygote

pleiotropy

a single gene affects multiple traits

recessive lethal allele

An allele that causes death when homozygous

typical offspring ratio with recessive lethal allele

2:1

cause of 2:1 phenotypic ratio

one genotype class is missing due to lethality

dominant inheritance pattern

trait appears in every generation

recessive inheritance pattern

trait can skip generations

carrier

an individual heterozygous for a recessive allele who does not show the phenotype

probability an unaffected child of two carriers is a carrier

2/3

1:1:1:1 ratio indicates

dihybrid testcross

3:1 ratio indicates

complete dominance in monohybrid cross

1:2:1 ratio indicates

incomplete dominance or codominance

2:1 ratio indicates

lethal allele involved